This invention relates to a light emitting device comprising a light emitting diode or laser diode (LED) and an excitable phosphor, which surrounds the LED chip. The phosphor is contained within a light transmissive medium of variable thickness for a more uniform emission of the light output. The invention finds particular application in combination with a UV/Blue LED and a phosphor or blend of phosphors, for converting LED-generated ultraviolet (UV) and/or blue light into white light for general illumination purposes. It should be appreciated, however, that the invention is also suitable to the conversion of light from other light sources to light of a different wavelength.
The advent of GaN-based epitaxial structures has lead to the development of UV and/or blue (“UV/blue”) light emitting devices, including light emitting diodes and lasers (both are generally referred to herein as LED's). By combination of the light emitting device with one or more phosphors, generation of visible light (e.g., red, blue, or white light) is achieved. The phosphor transforms a portion of the UV and/or blue light into light of longer wavelength, for example, by employing a UV/blue absorbing, yellow emitting phosphor, such as Y3Al5O12—Ce3+ (commonly referred to as YAG-Ce).
To form white light, the YAG-Ce phosphor converts a portion of the LED UV/blue light into yellow light. This produces a white field with a color rendering index (CRI) of about 77 and a color temperature ranging from about 6000 K to 8000 K. For some applications, conversion of UV/blue light from an LED to visible light using phosphors may be more attractive than the direct application of visible LEDs comprising combined LEDs emitting in several color bands to yield collectively a white light. Such UV/blue LED phosphor devices, for example, offer the opportunity to encompass a wider color range, which is important for display as well as for illumination applications. Similarly, the color of the light can be modified by combining two or more phosphors.
LED's, including the blue and UV emitting types, comprise a generally rectangular chip or die, formed from a semiconductor material, that radiates in a non-uniform fashion. In particular, the intensity of light generated at some regions of the front surface of the attached die (e.g., at the bond pads) is only a small fraction of the light emitted from the translucent sides and the remainder of the front (or back, in the case of a flip chip). When the die is coated with a layer of phosphor, the non-uniformity in the radiation emitted by the LED results in non-uniform excitation of the phosphor and may cause non non-uniformity in the color and/or intensity of the light emitted by the device.
With phosphor coated visible LED's, too thick of a phosphor coating over the less light intense areas reduces the light output and shifts the color of the output in those regions by not allowing enough of the LED wavelength emission to pass through to balance the color from the phosphor emission. This results in an overall emission which is not “white,” since it is dominated by the phosphor emission color.
Alternatively, too thin of a coating over the more intense areas of the LED allows too much of the LED wavelength emission to pass through, resulting in light that is not white, since it is dominated by the LED color. In the case of a UV-emitting LED, harmful UV rays may be emitted from the lamp, or must be stopped by a UV-reflective coating on the lens, if the phosphor layer is too thin to absorb them.
Thus, a uniform coating may be ideal for some areas of the die, but too thick or too thin for others, resulting in non-uniform color emission as well as reduced efficiency.
Another problem with LED emission is that the die attach materials tend to block light emitted from the sides of the LED. These materials are used to attach the LED to a cup or other support and to provide thermal transfer away from the die to a heat sink. The typical die attach materials used are silver epoxy or ceramic filled epoxy, which are thermally cured. During the die attach process, there is an inherent wicking of the attach material up the sides of the die. It is difficult to apply a sufficient amount of the die attach material to bond the device in place and provide thermal conduction without some wicking taking place. Particularly in the case of LED's with sapphire substrates, this causes significant degradation in the light output, since a large proportion of the light output from such LED's is through the sides.
The present invention provides a new and improved light source, and method of formation, which overcomes the above-referenced problems and others.